A scale assembly is provided with a scale which has a predetermined scale increment. The scale assembly consists of a plurality of scale sections, each of which is provided with a length portion of said scale and which are separated by joints having in the longitudinal direction of said scale a width that is smaller than a predetermined width. A measuring slide is movable along said scale and carries first and second photoelectric sensor means, which are spaced apart along said scale by a distance which exceeds said predetermined width. Each of said sensor means are adapted to be enabled and when enabled are adapted to photoelectrically scan said scale and to deliver a predetermined number of length signals in response to the movement of said slide along said scale to the extent of each of said increments. Integrating means are provided for integrating each of said length signals from each of said sensor means which are enabled. A change-over switch is shiftable between first and second states and arranged to enable said first and second sensor means in said first and second positions, respectively. Control means are carried by said slide and arranged to shift said change-over switch from either of said states to the other in response to the movement of said sensor means which are enabled at a time to one of said joints.